Pros, Cons of Tinkering with Plants’ Light-Harvesting Capacity

We all learned about photosynthesis in school: Plants convert light energy into chemical energy, and that fuels most of the planet’s living beings.

Yes… but not quite.

Beronda Montgomery, professor at the MSU-DOE Plant Research Laboratory, has published an article in the journal Algal Research – in collaboration with the Kramer Lab – that delves into how photosynthesis has evolved differently in nature, particularly in cyanobacteria.

Humans use plants for leisure, fuel, and food, and all these functions depend on plants’ ability to photosynthesize in order to grow. We already know that plants do not capture most of the light they are exposed to, but they grow well with what they get. The PRL is looking for ways to increase plant light-harvesting capacity, which would theoretically increase harnessable energy for our own uses without overloading the plants. But Montgomery’s research shows how photosynthesis is complicated to manipulate.

How cyanobacteria captures light

Light travels in waves of different lengths, each wavelength perceived as a unique color when reflected on surfaces. For example, a red shirt reflects red waves and absorbs all the others in the light spectrum; that is why we see “red” when the reflected light reaches our eyes. White light, such as sunlight, reflects all colors in the spectrum; black absorbs them all.

Plant growth is affected by such light quality, and Montgomery has focused on how cyanobacteria properly capture available light in changing environments.

Many cyanobacteria live in water, alternatively sinking or floating. While surface water is rich in red light, deeper levels have abundant green light and scarcer red light. To adapt to these different environments, the cyanobacteria cell produces Phycobilisomes (PBS), made of stacks of donut-like proteins that act as light-harvesting antennae. These proteins specialize in different wavelengths: some capture red light, others green light, and so on.

Each environment requires a unique protein mix for maximum light capture, and any changes in light quality affects that mix. To over simplify, the cyanobacteria might need a majority of red capturing proteins when close to the surface, but as the organism goes deeper – a process which can take up to two or three days – it will eventually need 75% green capturing proteins and 25% red as it adapts to the new surroundings.

Producing these proteins uses up energy. If they are unused, they are discarded. In that case, the energy that went into their production is considered wasted, as it could have been used elsewhere.

That is exactly what happens as cyanobacteria adapt protein mixes to new light conditions: formerly useful proteins are replaced with new and better suited ones. Transitions are costly propositions.

The pros and cons of light capture

Montgomery’s study examined the pros and cons of building and changing PBS and how costly transitions really are. Montgomery used a cyanobacteria mutant, deficient in PBS such that it did not adapt to light changes. That allowed for comparisons with the normal wild type (WT) under two sets of light conditions: constant, that mimic the gradual passing hours of the day, and fluctuating, that mimic sinking, floating, or environmental situations such as cloudy skies or water mixing.

Under the constant light conditions, WT grew faster than the mutants, due to the gradual adaptations to the “latest” light surroundings.

Highly fluctuating light conditions were quite costly to the WT due to the constant building of new proteins and the damage resulting from protein mismatch with external light availability (That mismatch produced reactive oxygen species, the same molecules that contribute to cell damage and aging in humans). WT had longer stacks of PBS, indicating the need for abundant proteins to adapt to these uncertain transitional situations – much like a satellite dish needing to be bigger when the signal is weak.

The mutant fared well against the WT in highly fluctuating light conditions, as it comparatively spent much less energy due to its inability to adapt to surrounding changes.

These findings confirm observations in nature. The WT strain dominates, since long-term and stable light situations are maintained for a longer portion of the organisms’s life cycle. Fluctuations are comparatively short-term events.

Looking towards a green economy

This fundamental research sheds a light on the evolution of variations in light capturing abilities. (Another twist from the study: species similar to the mutant do naturally exist in the world, although in much smaller numbers.) Although similar studies have been conducted on various cyanobacteria species, this is the first to compare one that adapts to different wavelengths.

Why cyanobacteria? These organisms, which have higher photosynthetic efficiency than land plants, have been recently examined as potential sources of renewable energy and other useful compounds. Increasing their energy output would take us a step closer to bringing biofuels to the market. Various PRL labs are working precisely on this issue, from the Ducat lab’s cyanobacteria harvesting research, to the Kerfeld lab’s creation of miniature energy factories in a lab environment, to the Kramer’s lab use of big data towards understanding photosynthetic mechanisms, only to name a few projects.

Coaxing cyanobacteria into accepting more light might enhance the coming of green economies, but Montgomery’s insights indicate that perhaps what we consider a currently missed opportunity is precisely what keeps these organisms balanced and alive.